FR2998584A1 - ELECTROLYSER COMPRISING ANIONICALLY CONDUCTIVE ELECTROLYTIC MEMBRANE AND METHOD OF ELECTROLYSIS OF WATER VAPOR - Google Patents

Abstract

The present invention relates to an anionic conduction electrolyser comprising an electrochemical cell (10) formed by a solid electrolyte (13) formed by a ceramic; a first electrode (11) forming the anode of the electrochemical cell (10); a second electrode (12) forming the cathode of said electrochemical cell (10), said solid electrolyte (13) being disposed between said first electrode (11) and said second electrode (12); said electrolyser being characterized in that said solid electrolyte (13) is made of zirconium oxide doped with erbium oxide.

Description

ELECTROLYSER COMPRISING ANIONICALLY CONDUCTIVE ELECTROLYTIC MEMBRANE AND METHOD OF ELECTROLYSIS OF WATER VAPOR.

TECHNICAL FIELD The field of the invention is that of electrolysis devices such as high temperature electrolysers comprising an anionic conduction membrane for generating hydrogen and oxygen adsorbates by electrolysis of water vapor to produce absence of other hydrogen and oxygen gas or respectively to hydrogenate, or to oxygenate cathode or anode compartments of the electrolyser. STATE OF THE ART The current technologies for high temperature electrolyzers, for example of the SOEC (Solid Oxyd Electrolyser Cell) type, are based on the use of two electrically conductive electrodes, separated by an electrolyte with an insulating electronic membrane and an ionic conductor. (anionic), and separating the gases from the anode and cathode compartments thus forming a structure called an electrochemical cell or elementary assembly.

Solid electrolytes with anionic conduction are generally a ceramic membrane formed by an oxide-doped zirconium oxide (or zirconia) belonging to the Lanthanide chemical series of degree of oxidation 3+ (less than the degree 4 of the zirconium ion ( Substituted Zr4 +). The solid conducting electrolytes of O 2 - ions are commonly composed of zirconium oxide stabilized by an yttrium oxide (Y 2 O 3), the yttrium oxide content being adjusted so as to obtain ionic conduction and optimum mechanical stability. However, the electrolysers, whose membrane is formed by a solid electrolyte zirconia oxide stabilized with yttrium oxide, have a limited life during use under water vapor, and quickly begin a decline from 1000 hours of operation. This limited life is mainly due to the decrease of the conductivity of the membrane during its use. This decrease in the ionic conductivity of the membrane is induced by the modification of its crystallographic structure. Indeed, during prolonged use under water vapor, zirconia doped yttrium oxide stabilized in cubic symmetry will change structure through a quadratic symmetry structure, then monoclinic, which will lead in a first time delamination problems electrolyte interfaces / electrodes with a loss of electrochemical performance then in a second time a rupture of the interfaces of the electrolyte membrane, the zirconia being passed in the form of a powder in the monoclinic state, this phenomenon being all the more accentuated if the electrochemical cell is under water vapor pressure.

Zirconia-based solid electrolytes doped with an ytterbium oxide (Yb203) are also known. However, ytterbium oxide has two degrees of oxidation (2 and 3) which goes from degree 3 to degree 2 in a reducing medium which leads to an increase in its ionic radius from Yb3 + (degree of oxidation 3). at Yb2 + (oxidation state 2). The modification of the size of the dopant, as a substitute for zirconia in its cubic structure, therefore causes a modification of the crystallographic stability of the cubic structure leading to a demixing phenomenon of the structure forming a phase of Yb203.

Thus, in operation under a water vapor atmosphere, such an electrolytic membrane also exhibits delamination phenomena of the electrolyte / electrode interfaces. Thus, the solid electrolytes used in the context of an application of anionic conduction water vapor electrolysis have a limited lifetime which is a brake on the development of these technologies. SUMMARY OF THE INVENTION In this context, the invention aims at providing an anionic conduction electrolyser for use under water vapor comprising an electrochemical cell making it possible to overcome the phenomena of demixing and delamination of the electrolyte / electrode interfaces. the electrochemical cell. To this end, the invention provides an anionic conduction electrolyser comprising an electrochemical cell formed by: a solid electrolyte formed by a ceramic; a first electrode forming the anode of the electrochemical cell; a second electrode forming the cathode of said electrochemical cell, said solid electrolyte being disposed between said first electrode and said second electrode; said electrolyser being characterized in that said solid electrolyte is made of zirconium oxide doped with erbium oxide. The electrochemical cell according to the invention makes it possible to overcome the phenomenon of delamination of the interfaces of the electrode / electrolyte / electrode assembly and the demixing phenomenon of the solid electrolyte related to the conditions of use under water of the assemblies during the production of hydrogen and oxygen. The erbium oxide is advantageously chosen as a dopant because it has a single degree of oxidation of degree 3 (close to the Zr4 + ion) and a size (ionic radius) close to the size of the substituted Zr4 + ion. Thus, in reducing medium (under H2) and at high temperature (between 600 ° C and 1000 ° C), the dopant will remain stable since its size and degree of oxidation will not change. The use of erbium oxide as a dopant makes it possible to avoid the demixing phenomena due to the instability of the structure of the electrochemical cell and in particular of the structure of zirconia oxide doped in the cubic or quadratic form in medium oxidant. The anionically conductive electrolyser comprising an electrochemical cell according to the invention may also have one or more of the following characteristics, considered individually or in any technically possible combination: said solid electrolyte is formed by a zirconium oxide ceramic doped between 8 and 15% of erbium oxide; said first electrode and / or said second electrode is a metal electrode; said first electrode and / or said second electrode is a ceramic electrode; said first electrode and / or said second electrode is formed by a cermet; said cermet consists of a mixture of a ceramic and an electrically passivable alloy capable of forming a passive layer; the nature and the metal element content of said passivable alloy being determined so that the melting temperature of said alloy is greater than the sintering temperature of said electrolyte membrane; said electrochemical cell has an electrode / electrolyte / symmetrical electrode assembly.

The subject of the invention is also a method for generating hydrogen and oxygen by electrolysis of high temperature steam by means of an electrolyzer according to the invention comprising an anionic conduction solid electrolyte, said electrolyte being disposed between a first electrode forming the anode of the electrolyser and a second electrode forming the cathode of the electrolyser, said method comprising the following steps: - insertion of water in the form of steam introduced under pressure at the cathode; circulation of a current between the anode and the cathode; - Generation of vacancies Vo ** in the solid electrolyte following oxidation in the anode compartment with oxygen generation; migration of said vo vacancies in the solid electrolyte; - Reducing said water introduced in vapor form at the cathode; generating hydrogen at the cathode following said reduction; said solid electrolyte being made of zirconium oxide doped with erbium oxide.

Advantageously, the process for generating hydrogen and oxygen by electrolysis of water vapor characterized in that said water vapor is electrolyzed between 600 ° C and 1000 ° C. BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will emerge more clearly from the description which is given below, by way of indication and in no way limiting, with reference to the attached figure schematically showing a sectional view of a electrochemical cell according to the invention. DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT With reference to the single figure, the electrochemical cell 10, also called elementary assembly, is formed by a solid electrolyte 13, or electrolyte membrane, with anionic conduction lined on either side by electrodes 11 and 12 (the anode and the cathode respectively). The solid electrolyte 13 is made of zirconium oxide doped with erbium oxide. Erbium is a dopant whose size (i.e. the ionic radius) is close to that of zirconium.

Indeed, to be a good substituent, the dopant must have, for the coordination 7 or 8, a size comparable to the substituted zirconia. Thus, in coordination 8, the ionic radius of the Zr4 + ion (R = 98 picometer (μm) is close to that of erbium oxide (Er3 +) (R = 114.4 μm). Erbium oxide has the advantage of being electrochemically stable in a water vapor medium since it has only one degree of oxidation (3+), so that the crystallographic structure of the doped ceramic remains stable. at high temperature (between 600 ° C. and 1000 ° C.), the solid electrolyte is advantageously formed by a zirconium oxide ceramic doped with between 8 and 15% of erbium oxide. The electrodes 11, 12 may be metal electrodes or else ceramic electrodes, having an electronic / ionic mixed conduction. According to another embodiment of the invention, the electrodes 11, 12 of the cell 10 are formed by a cermet constituted by a mixture of a ceramic (advantageously identical to the ceramic of the electrolyte 2 9985 84 7 soli de) and a stainless steel According to another embodiment of the invention, the electrodes 11, 12 of the cell 10 are formed by a cermet constituted by a mixture of a ceramic (advantageously identical to the ceramic of the solid electrolyte) and a transition metal such as Chromium (Cr), Cobalt (Co), or Nickel (Ni). According to another embodiment of the invention, the electrodes 11, 12 of the cell 10 are formed by a cermet constituted by a mixture of a ceramic (advantageously identical to the ceramic of the solid electrolyte) and a passivable metal electrically conductive capable of forming a protective oxide layer so as to protect it in an oxidizing environment (ie at the anode of the electrolyser). The passivable alloy comprises, for example, chromium (which is a transition element) so as to have a cermet having the particularity of not being oxidized at temperature or elements such as cobalt, nickel or a noble metal such as gold, platinum or palladium. According to another embodiment of the invention, oxidizable refractories may also be formed based on titanium, zirconium, niobium, tantalum, or tungsten.

The composition of the cermet is determined according to the temperature of use. Thus, for example, above 700 ° C., it is preferable to use Crofer 22 APU-type chrome cermets. Advantageously, the electrochemical cell 10 has a symmetrical electrode / electrolyte / electrode assembly. The symmetry of the electrochemical cell 10 has the advantage of reducing the number of steps in the implementation of the method of manufacturing such cells, but also of being able to reversibly use the electrochemical cell.

The symmetry of the electrochemical cell also makes it possible to reduce manufacturing costs, particularly if the manufacturing process is preferably a co-pressing or co-sintering process. Such a solid electrolyte based on zirconium doped with erbium oxide thus makes it possible to improve the lifetime of the electrolyte and consequently of the electrolyzer capable of generating hydrogen and oxygen by injecting steam. pressurized water. Such a method for generating hydrogen and oxygen by electrolysis of high temperature steam by means of the electrolyser 10 according to the invention comprises the following steps: - insertion of water in the form of steam introduced under pressure to the cathode; circulation of a current between the anode and the cathode; generation of vo vacancies in the solid electrolyte following oxidation in the anode compartment with oxygen generation; migration of said vo vacancies in the solid electrolyte; - Reducing said water introduced in vapor form at the cathode; generation of hydrogen at the cathode following said reduction; said solid electrolyte being made of zirconium oxide doped with erbium oxide. Advantageously, the process for generating hydrogen and oxygen by electrolysis of water vapor is carried out at high temperature, that is to say between 600 ° C and 1000 ° C. The invention has been particularly described for the generation of hydrogen and oxygen by electrolysis of steam; however, the membrane according to the invention can also be used in the field of fuel cells or in the field of the treatment of carbonaceous gases (CO2, CO) by electrochemical hydrogenation.

Claims (9)

REVENDICATIONS1. Anionically conductive electrolyser comprising an electrochemical cell (10) formed by: - a solid electrolyte (13) formed by a ceramic; a first electrode (11) forming the anode of the electrochemical cell (10); a second electrode (12) forming the cathode of said electrochemical cell (10), said solid electrolyte (13) being disposed between said first electrode (11) and said second electrode (12); said electrolyser being characterized in that said solid electrolyte (13) is made of zirconium oxide doped with erbium oxide. 2. anionic conduction electrolyser according to the preceding claim characterized in that said solid electrolyte (13) is formed by a zirconium oxide ceramic doped between 8 and 15% erbium oxide. 3. anionic conduction electrolyser according to one of the preceding claims characterized in that said first electrode (11) and / or said second electrode (12) is a metal electrode. 4. anionic conduction electrolyser according to one of the preceding claims characterized in that said first electrode (11) and / or said second electrode (12) is a ceramic electrode. 5. anionic conduction electrolyser according to one of the preceding claims characterized in that said first electrode (11) and / orladite second electrode (12) is formed by a cermet. 6. anionic conduction electrolyser according to the preceding claim characterized in that said cermet is constituted by the mixture of a ceramic and a passivable alloy conductive electron capable of forming a passive layer; the nature and the metal element content of said passivable alloy being determined so that the melting temperature of said alloy is greater than the sintering temperature of said electrolyte membrane. 7. anionic conduction electrolyser according to one of the preceding claims characterized in that said electrochemical cell (10) has a symmetrical electrode / electrolyte / electrode assembly. 8. A method for generating hydrogen and oxygen by electrolysis of high temperature steam by means of an electrolyzer according to one of claims 1 to 7 comprising an anionic conducting solid electrolyte (10), said electrolyte being disposed between a first electrode (11) forming the anode of the electrolyser and a second electrode (12) forming the cathode of the electrolyzer, said method comprising the following steps: insertion of water in the form of vapor introduced under cathode pressure; circulation of a current between the anode and the cathode; generation of vo vacancies in the solid electrolyte following oxidation in the anode compartment with oxygen generation; migration of said Vo vacancies in the solid electrolyte; reducing said water introduced in vapor form at the cathode; generating hydrogen at the cathode subsequent to said reduction; said solid electrolyte being made of zirconium oxide doped with erbium oxide. 9. A method of generating hydrogen and oxygen by electrolysis of water vapor according to claim 8 characterized in that said process is carried out at a temperature between 600 ° C and 1000 ° C.